The invention relates to a control system for use in fractional distillation columns. More specifically the invention relates to a control system for use in a dividing wall fractionation column.
Fractional distillation is a well developed unit operation used in the petrochemical, chemical and petroleum refining industries to separate volatile chemical compounds. The operation of fractionation columns is regulated by control systems monitoring such variables as temperature, liquid levels and fluid flow rates. The control systems need to change operation of the column as by increasing the rate of heat input or decreasing the rate at which a stream is removed to compensate for occasional or periodic changes in the composition of the feed stream to the fractionation column or other factors which can affect the operation of the column.
Fractionation columns are widely employed to do myriad separations in a number of industries. Control systems for fractionation columns have therefore reached a high state of development including the use of on-line analytical instruments and computerized optimization. They still, however, rely to a great extent on the measurement of temperatures and liquid levels in the fractionation column by equipment such as shown in U.S. Pat. No. 3,855,074 issued to H. A. Mosler et. al.
The dividing wall or Petlyuk configuration for fractionation columns was initially introduced some 50 years ago by Petlyuk et al. Dividing wall columns have been employed for the separation of hydrocarbon mixtures as evidenced by the disclosure of U.S. Pat. No. 2,471,134 issued to R. O. Wright. Recently the use of dividing wall columns has begun to expand because of the greater recognition that in certain situations dividing wall columns can provide benefits above those of conventional fractionation columns. For instance, a commercialization of a fractionation column employing this technique is described in the article Thermal Coupling for Energy Efficiency appearing at page s14 of a supplement to The Chemical Engineer, Aug. 27, 1992.
Control systems for dividing wall columns are not as mature or as commonly described as for conventional columns. Studies of control variables and responses in a small scale pilot plant column are given in Operation and Control of Dividing Wall Distillation Columns, by A. Mutalib and R. Smith, Part 1: Degrees of Freedom and Dynamic Simulation, Trans. IChemE. Vol. 76, Part A, March 1998, pages 308-318 and Part 2: Simulation and Pilot Plant Studies Using Temperature Control, Trans. IChemE, Vol 76, Part A, March 1998, pages 319-334. Two other papers directed to the general control of dividing wall columns are The Control of Dividing Wall Column (Centre for Process Integration, UMIST, Manchester, UK) by F. Lestak and R. Smith appearing at page 307 of Chemical Engineering Research and Design, Institution of Chemical Engineers (1993) 71 (A3) and Optimizing Control of Petlyuk Distillation: Understanding the Steady-State Behavior by Ivar J. Halvorsen and Sigurd Skogestad at pages s249-s254 of Computers Chem. Engng., Vol. 21, Suppl., 1997 (Elsevier Science Ltd.)
An example of an actual control system for a dividing wall column is provided in U.S. Pat. No. 4,230,533 issued to V. A. Giroux. This reference is also relevant as liquid is collected in an upper portion of the column and divided between the two sides of the dividing wall by level control.
The invention is a method of controlling the operation of a dividing wall fractional distillation column. The liquid flowing downward in an upper portion of the column is collected and then divided by a ratio controller between parallel feed and product dividing wall sections. Temperature measurements taken in upper and lower portions of the column are used to control respectively the rate at which overhead liquid is returned to the column and the rate at which the sidedraw product is removed from the product dividing wall section.
One embodiment of the invention may be described as a dividing wall fractional distillation column comprising a vertical outer column having a vertical central portion divided into a feed section and a parallel product section by a dividing wall, the parallel feed and product sections having upper and lower ends, with the dividing wall column also comprising an upper section containing vapor-liquid contacting devices; an overhead condensing system in which vapor rising from the upper section of the column is at least partially condensed and an overhead liquid is produced; a first flow control valve which controls the flow of a portion of said overhead liquid into the upper section of the column as reflux; a first temperature sensor, which monitors the temperature in the upper end of the product section of the column and generates a signal used in controlling the operation of the first flow control valve; a first liquid collection system located in an upper portion of the column below the upper section and above the dividing wall, the first liquid collection system blocking downward liquid flow through substantially all of the cross section of the column; a flow ratio controller which controls the division of liquid collected in the first liquid collection system into separate streams which flow via a first and second conduits into the feed and product sections of the column; a second liquid collection system, which is located in an intermediate portion of product section of the column, with the second liquid collection system having a liquid collection well from which a side product is removed from the column; a second flow control valve which controls the rate of side product removal in response to a second signal; a second temperature sensor, which monitors the temperature in the lower end of the product section of the column and generates a signal used in controlling the operation of the second flow control valve; a control system regulating the heat input to the column by a reboiler system located at the bottom of the column; and, a flow control system regulating the rate at which a net bottoms product is removed from the column.